A number of liquefaction systems for cooling, liquefying, and optionally sub-cooling natural gas are well known in the art, such as the single mixed refrigerant (SMR) cycle, the propane-precooled mixed refrigerant (C3MR) cycle, the dual mixed refrigerant (DMR) cycle, C3MR-Nitrogen hybrid (such as AP-X™) cycles, the nitrogen or methane expander cycle, and cascade cycles. Typically, in such systems, natural gas is cooled, liquefied, and optionally sub-cooled by indirect heat exchange with one or more refrigerants. A variety of refrigerants might be employed, such as mixed refrigerants, pure components, two-phase refrigerants, gas phase refrigerants, etc. Mixed refrigerants (MR), which are a mixture of nitrogen, methane, ethane/ethylene, propane, butanes, and pentanes, have been used in many base-load liquefied natural gas (LNG) plants. The composition of the MR stream is typically optimized based on the feed gas composition and operating conditions.
The refrigerant is circulated in a refrigerant circuit that includes one or more heat exchangers and a refrigerant compression system. The refrigerant circuit may be closed-loop or open-loop. Natural gas is cooled, liquefied, and/or sub-cooled by indirect heat exchange in one or more refrigerant circuits by indirect heat exchange with the refrigerants in the heat exchangers.
The refrigerant compression system includes a compression sequence for compressing and cooling the circulating refrigerant, and a driver assembly to provide the power needed to drive the compressors. For precooled liquefaction systems, the quantity and type of drivers in the driver assembly and the compression sequence have an impact on the ratio of the power required for the precooling system and the liquefaction system. The refrigerant compression system is a critical component of the liquefaction system because the refrigerant needs to be compressed to high pressure and cooled prior to expansion in order to produce a cold, low pressure refrigerant stream that provides the heat duty necessary to cool, liquefy, and optionally sub-cool the natural gas.
DMR processes involve two mixed refrigerant streams, the first for precooling the feed natural gas and the second for liquefying the precooled natural gas. The two mixed refrigerant streams pass through two refrigerant circuits, a precooling refrigerant circuit within a precooling system, and a liquefaction refrigerant circuit within a liquefaction system. In each refrigerant circuit, the refrigerant stream is vaporized while providing the cooling duty required to cool and liquefy the natural gas feed stream. When a refrigerant stream is vaporized at a single pressure level, the system and process is referred to as “single pressure”. When a refrigerant stream is vaporized at two or more pressure levels, the system and process is referred to as “multiple pressure”. Referring to FIG. 1, a DMR process of the prior art is shown in cooling and liquefaction system 100. The DMR process described herein involves a single pressure liquefaction system and a multiple pressure precooling system with two pressure levels. However, any number of pressure levels may be present. A feed stream, which is preferably natural gas, is cleaned and dried by known methods in a pre-treatment section (not shown) to remove water, acid gases such as CO2 and H2S, and other contaminants such as mercury, resulting in a pretreated feed stream 102. The pretreated feed stream 102, which is essentially water free, is precooled in a precooling system 134 to produce a second precooled natural gas stream 106 and further cooled, liquefied, and/or sub-cooled in a main cryogenic heat exchanger (MCHE) 164 to produce a first LNG stream 108. The first LNG stream 108 is typically let down in pressure by passing it through an LNG pressure letdown device 111 to produce a reduced pressure LNG stream 103, which is then sent to a flash drum 107 to produce a flash gas stream 109 and a second LNG stream 105. The second LNG stream 105 may be let down to storage pressure and sent to an LNG storage tank (not shown). The flash gas stream 109 and any boil-off gas (BOG) produced in the storage tank may be used as fuel in the plant and/or sent to flare.
The pretreated feed stream 102 is cooled in a first precooling heat exchanger 160 to produce a first precooled natural gas stream 104. The first precooled natural gas stream 104 is cooled in a second precooling heat exchanger 162 to produce the second precooled natural gas stream 106. The second precooled natural gas stream 106 is liquefied and subsequently sub-cooled to produce the first LNG stream 108 at a temperature between about −170 degrees Celsius and about −120 degrees Celsius, preferably between about −170 degrees Celsius and about −140 degrees Celsius. MCHE 164 shown in FIG. 1 is a coil wound heat exchanger with two tube bundles, a warm bundle 166 and a cold bundle 167. However, any number of bundles and any exchanger type may be utilized. Although FIG. 1 shows two precooling heat exchangers and two pressure levels in the precooling circuit, any number of precooling heat exchangers and pressure levels may be utilized. The precooling heat exchangers are shown to be coil wound heat exchangers in FIG. 1. However, they may be plate and fin heat exchangers, shell and tube heat exchangers, or any other heat exchangers suitable for precooling natural gas.
The term “essentially water free” means that any residual water in the pretreated feed stream 102 is present at a sufficiently low concentration to prevent operational issues associated with water freeze-out in the downstream cooling and liquefaction process. In the embodiments described herein, water concentration is preferably not more than 1.0 ppm and, more preferably between 0.1 ppm and 0.5 ppm.
The precooling refrigerant used in the DMR process is a mixed refrigerant (MR) referred to herein as warm mixed refrigerant (WMR) or “first refrigerant”, comprising components such as nitrogen, methane, ethane/ethylene, propane, butanes, and other hydrocarbon components. As illustrated in FIG. 1, a low pressure WMR stream 110 is withdrawn from the warm end of the shell side of the second precooling heat exchanger 162 and compressed in a first compression stage 112A of a WMR compressor 112. A medium pressure WMR stream 118 is withdrawn from the warm end of the shell side of the first precooling heat exchanger 160 and introduced as a side-stream into the WMR compressor 112, where it mixes with the compressed stream (not shown) from the first compression stage 112A. The mixed stream (not shown) is compressed in a second WMR compression stage 112B of the WMR compressor 112 to produce a compressed WMR stream 114. Any liquid present in the low pressure WMR stream 110 and the medium pressure WMR stream 118 is removed in vapor-liquid separation devices (not shown).
The compressed WMR stream 114 is cooled and preferably condensed in WMR aftercooler 115 to produce a first cooled compressed WMR stream 116, which is introduced into the first precooling heat exchanger 160 to be further cooled in a tube circuit to produce a second cooled compressed WMR stream 120. The second cooled compressed WMR stream 120 is split into two portions: a first portion 122 and a second portion 124. The first portion of the second cooled compressed WMR stream 122 is expanded in a first WMR expansion device 126 to produce a first expanded WMR stream 128, which is introduced into the shell side of the first precooling heat exchanger 160 to provide refrigeration duty. The second portion of the second cooled compressed WMR stream 124 is introduced into the second precooling heat exchanger 162 to be further cooled, after which it is expanded in a second WMR expansion device 130 to produce a second expanded WMR stream 132, which is introduced into the shell side of the second precooling heat exchanger 162 to provide refrigeration duty. The process of compressing and cooling the WMR after it is withdrawn from the precooling heat exchangers is generally referred to herein as the WMR compression sequence.
Although FIG. 1 shows that compression stages 112A and 112B are performed within a single compressor body, they may be performed in two or more separate compressors. Further, intermediate cooling heat exchangers may be provided between the stages. The WMR compressor 112 may be any type of compressor such as centrifugal, axial, positive displacement, or any other compressor type.
In the DMR process, liquefaction and sub-cooling is performed by heat exchanging precooled natural gas against a second mixed refrigerant stream, referred to herein as cold mixed refrigerant (CMR) or “second refrigerant”.
A warm low pressure CMR stream 140 is withdrawn from the warm end of the shell side of the MCHE 164, sent through a suction drum (not shown) to separate out any liquids and the vapor stream is compressed in CMR compressor 141 to produce a compressed CMR stream 142. The warm low pressure CMR stream 140 is typically withdrawn at a temperature at or near WMR precooling temperature and preferably less than about −30 degree Celsius and at a pressure of less than 10 bara (145 psia). The compressed CMR stream 142 is cooled in a CMR aftercooler 143 to produce a compressed cooled CMR stream 144. Additional phase separators, compressors, and aftercoolers may be present. The process of compressing and cooling the CMR after it is withdrawn from the warm end of the MCHE 164 is generally referred to herein as the CMR compression sequence.
The compressed cooled CMR stream 144 is then cooled against evaporating WMR in precooling system 134. The compressed cooled CMR stream 144 is cooled in the first precooling heat exchanger 160 to produce a first precooled CMR stream 146 and then cooled in the second precooling heat exchanger 162 to produce a second precooled CMR stream 148, which may be fully condensed or two-phase depending on the precooling temperature and composition of the CMR stream. The CMR stream 148 is then liquefied and/or subcooled in the liquefaction system 165. FIG. 1 shows an arrangement wherein the second precooled CMR stream 148 is two-phase and is sent to a CMR phase separator 150 to produce a CMR liquid (CMRL) stream 152 and a CMR vapor (CMRV) stream 151, which are both sent back to the MCHE 164 to be further cooled. Liquid streams leaving phase separators are referred to in the industry as MRL and vapor streams leaving phase separators are referred to in the industry as MRV, even after they are subsequently liquefied.
Both the CMRL stream 152 and CMRV stream 151 are cooled in two separate circuits of the MCHE 164. The CMRL stream 152 is cooled in a warm bundle 166 of the MCHE 164, resulting in a cold stream that is let down in pressure across CMRL expansion device 153 to produce an expanded CMRL stream 154, that is sent back to the shell side of MCHE 164 to provide refrigeration required in the warm bundle 166. The CMRV stream 151 is cooled in the warm bundle 166 and subsequently in a cold bundle 167 of MCHE 164, then reduced in pressure across a CMRV expansion device 155 to produce an expanded CMRV stream 156 that is introduced to the MCHE 164 to provide refrigeration required in the cold bundle 167 and warm bundle 166.
MCHE 164 and precooling heat exchanger 160 can be any exchanger suitable for natural gas cooling and liquefaction such as a coil wound heat exchanger, plate and fin heat exchanger, or a shell and tube heat exchanger. Coil wound heat exchangers are the state of the art exchangers for natural gas liquefaction and include at least one tube bundle comprising a plurality of spiral wound tubes for the flowing process and warm refrigerant streams and a shell space for flowing a cold refrigerant stream.
In the arrangement shown in FIG. 1, the cold end of the first precooling heat exchanger 160 is at a temperature below 20 degrees Celsius, preferably below about 10 degrees Celsius, and more preferably below about 0 degrees Celsius. The cold end of the second precooling heat exchanger 162 is at a temperature below 10 degrees Celsius, preferably below about 0 degrees Celsius, and more preferably below about −30 degrees Celsius. Therefore, the second precooling heat exchanger is at a lower temperature than the first precooling heat exchanger.
A key benefit of a mixed refrigerant cycle is that the composition of the mixed refrigerant stream can be optimized to adjust cooling curves in the heat exchanger and the outlet temperature, to increase the process efficiency. This may be achieved by adjusting the composition of the refrigerant stream for the various stages of the cooling process. For instance, a mixed refrigerant with a high concentration of ethane and heavier components is well suited as a precooling refrigerant while one with a high concentration of methane and nitrogen is well suited as a subcooling refrigerant.
In the arrangement shown in FIG. 1, the composition of the first expanded WMR stream 128 providing refrigeration duty to the first precooling heat exchanger is the same as the composition of the second expanded WMR stream 132 providing refrigeration duty to the second precooling heat exchanger 162. Since the first and second precooling heat exchangers cool to different temperatures, using the same refrigerant composition for both exchangers is inefficient. Further, the inefficiency increases with three or more precooling heat exchangers.
The reduced efficiency leads to an increased power required to produce the same amount of LNG. The reduced efficiency further results in a warmer overall precooling temperature at a fixed amount of available precooling driver power. This shifts the refrigeration load from the precooling system to the liquefaction system, rendering the MCHE larger and increasing the liquefaction power load, which may be undesirable from a capital cost and operability standpoint.
One approach to solving this problem is to have two separate closed loop refrigerant circuits for each stage of precooling. This would require separate mixed refrigerant circuits for the first precooling heat exchanger 160 and the second precooling heat exchanger 162. This would allow the compositions of the two refrigerant streams to be optimized independently and therefore improve efficiency. However, this approach would require separate compression systems for each precooling heat exchanger, which would lead to increased capital cost, footprint, and operational complexity, which is undesirable.
Another problem with the arrangement shown in FIG. 1 is that the power required by the precooling and liquefaction systems may not be equal, requiring a different number of drivers to provide the power. Often the liquefaction system has a higher power requirement than the precooling system due to typical precooling temperatures achievable. In some cases, it may be preferable to achieve a 50-50 power split between precooling and liquefaction system drivers.
Therefore, there is a need for an improved system for liquefying natural gas that provides more balance between the power requirements of the precooling and liquefaction systems and improving the efficiency of both systems, while avoiding an increase in capital cost, footprint or operational complexity.